The invention relates to a power controller for setting the power of the electrical loads of an electrical appliance, which is connected at least by two phases to a three-phase mains power supply, in particular for the electromechanically pulsed hot plates of electric cookers.
In electromechanical power controllers, a heated bimetallic strip acts on a snap-action switch, and the duty ratios in the individual power levels are set by a control cam.
In the case of those power controllers which use a heated bimetallic strip with a compensation bimetallic strip, the problem of switching hysteresis (which is linked to the bimetallic strip) arises, so that known power controllers can control the power only from about 7% to full load. However, setting to an even lower percentage value is desirable.
In order to overcome the switching hysteresis in an electromechanically pulsed power controller and to reduce the minimum available heating power, it is known from EP-A-0 562 287 for a region with a power level which is between the minimum power level and the maximum power level to be inserted in the switching region of the actuating element between the switched-off position and the minimum power level. As a result of the fact that the power is briefly increased to about 30 to 50% of the maximum power in the intermediate region, the pulsating contact of the snap-action switch remains ensured during the subsequent switching-back process, and the adjustment problem which occurs with such power controllers is solved by the bimetallic strip being briefly initially heated before the lowest power level is switched on.
In contrast to electromechanical power controllers, in the case of electronic power controllers which operate with a microprocessor which predetermines the duty ratio in the individual power levels, the power level can be set safely and infinitely variably between 0 and 100%. The disadvantage of these electronic power controllers is that the costs are several times the costs of an electromechanical power controller.
The invention is thus based on the object of providing a power controller having electromechanical pulsing, which is able to set low power levels reliably and reproducibly.
The invention can be applied in particular to electric cookers. As a rule, such an electric cooker is always connected to the three phases of a three-phase mains power supply. The first phase voltage and the second phase voltage are required for the four hot plates, while the third phase voltage is applied to the oven. Since not only the phase voltage of 230 V but also the line voltage of 400 V from the three-phase mains power supply is also available in such a circuit arrangement, a lower percentage power level can be made available safely and reproducibly by means of the connection according to the invention.
If the power requirement is high, for example for griddling or for boiling, the hot plates are switched to 400 V, while in the simmering and warming region, where less power is required, the hot plates are connected to 230 V.
If, for example, a hot plate with a heating resistance of 80xcexa9 is used, this results in a power level of 2000 W (100%) when connected to 400 V. If the same heating resistance is now connected to the phase voltage of 230 V, then the heating power is now U2/R=2302/80=661 W.
If the aim is now to achieve a 3% power level at the lowest setting, the power controller must be adjusted to about 9% at the lowest level. 9% of 661 W=59.5 W; 59.5 W of 2000 W=3%. In practice, the power controller would have to be set between 7.5 and 10.5%. This then results in 2.5 to 3.5% power at the lowest level.
Using this arrangement, it is thus possible to achieve a 3% power output at the lowest level even with an electromechanical power controller, which until now has been possible only using electronic power controllers.
The invention can also be applied to hot plates which are equipped with a cooking zone enlarged by a second heating circuit. With an appropriate switching arrangement, this second heating circuit can also be connected, optionally pulsed or unpulsed, to the line voltage of 400 V or to the phase voltage of 230 V.